Fixed fatal error in visit_profile_nodes() for DBI profiles with no Data.
Changed propagate_period_count() to not reset period_count.
Changed to increment period_count in end_sample_period() not start_sample_period().
Changed start_sample_period() to not call end_sample_period() automatically.
So now multiple start_sample_period() calls without end_sample_period() calls
are harmless - the period runs from the most recent start_sample_period() call.
Added period_strict_start and period_strict_end attributes to control how
missing/multiple start_sample_period() & end_sample_period calls are handled.
The attributes can be code refs which are called when needed.
Added extra_info attribute for use by scripts to carry any extra information.

Fixed DashProfiler::Import :optionally to still export stubs
Changed DashProfiler reset_all_profiles() to not call start_sample_period_all_profiles().
Changed DashProfiler::Auto to use file name not full path.
Changed DashProfiler::Core end_sample_period() to do nothing if no period is active.
Added more docs, especially to UserGuide and Auto.